Fine pitch finned tubing and method of producing the same



United States Patent 3,327,512 FINE PITCH FKNNED TUBING AND METHOD 0F PRODUCING THE SAME Arthur H. Novak, Royal Oak, and James G. Withers, Jr., Dearborn, Mich., assignors to Calumet & Hecla, Inc., Allen Park, Mich, a corporation of Michigan Filed Dec. 28, 1964, Ser. No. 421,390 10 Claims. c1. 72367) The present invention relates to fine pitch finned tubing and method of producing the same.

It is an object of the present invention to provide a method for producing finned tubing in which a finned section is interposed between two plain sections and in which the pitch of the fins in the finned section is relatively fine as for example between A and inch.

It is a further object of the present invention to provide a method of finning tubes by providing a finning roll composed of a plurality of discs of progressively increasing diameter from one end to the other, providing a relative feed between the roll and tube in a direction radially inwardly of the tube to move the roll to firming depth with respect thereto, providing continuous timed rotation of the roll and tube, maintaining the roll at a skew angle with respect to the tube so that the rolling contact between the roll and tube provides an axial movement of the tube, and increasing the skew angle continuously during the interval between initial contact between the roll and tube and the arrival of the roll at finning depth.

It is a further object of the present invention to provide a method as described in the preceding paragraph which comprises initially locating the roll at a taper angle at initial contact between the roll and tube such that the initial contact is between the disc of least diameter and the tube, and which comprises further the step of decreasing the taper angle continuously during the interval between initial contact between the roll and tube and arrival of the roll at finning depth.

It is a further object of the present invention to provide a method as described in the preceding paragraph in which the taper angle is reduced to an angle not substantially greater than zero when the roll is at finning depth.

It is a further object of the present invention to provide a finned tubing in which the fins have a height at least as great as the pitch, and in which the frequency of fins is at least 26 fins per inch.

Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying drawings, illustrating a preferred embodiment of the invention, wherein:

FIGURE 1 is an enlarged fragmentary sectional view illustrating the form and arrangement of the tube engaging portions of a roll.

FIGURE 2 is a plan view of a roll and tube in starting position.

FIGURE 3 is an elevational view of the roll and tube of FIGURE 2.

FIGURE 4 is a plan view of the roll and tube in rolling position.

FIGURE 5 is an elevational view of the roll and tube of FIGURE 4.

FIGURE 6 is a section of finned tubing produced in accordance with the present invention.

Integrally finned tubing has long been produced by an operation in which a tube is engaged by a plurality, usually three, finning rolls spaced uniformly about its periphery, each of the rolls being formed of a stack of discs of progressively increasing diameter positioned with their axes at skew angles with respect to the axis of the tube so that as the rolls are rotated, rotation and axial advance are transmitted to the tube and at the same time the maice terial of the tube wall is rolled into one or a plurality of continuous integral helically extending fins. It has long been recognized that the finer the pitch, the more difi'icult the manufacturing process becomes. In the interaction between the forming tools or rolls and the tube, accommV dations which are readily made for coarse pitch finning are not available in fine pitch finning.

In the past, integral finned tubing in which the fin pitch was approximately inch has been made commercially although the process was not economical, and at the present time such tubing is not offered commercially.

In accordance with the present invention fins having a pitch of down to inch become practical. While the present invention makes available for the first time a commercial offering of very fine pitch tubing, as for example from to inch, it nevertheless offers advantages in the formation of fins whose pitch is not as fine as the extremely fine pitch of the example. Actually, the present invention offers substantial commercial advantages in producing finned tubing in which the pitch is at least 15 fins per inch.

The present invention has particular application in the production of finned tubing of the type in which a finned section of tube is provided intermediate to adjacent plain or unfinned sections. In this type of tubing the production involves the feeding of the rolls radially inwardly into the tube to finning depth. This relative infeed is required to be somewhat gradual with the result that during the interval between initial contact of the rolls with the work piece and the arrival of the rolls at finning depth, there is produced on the work piece an imperfectly finned transition zone in which the fins are of gradually increasing height until at the end of the transition zone they reach full height and proper form.

With the fixed angle type of rolling generally used, consisting of a series of discs of progressively graduated diameters, there exists a certain amount of built-in misalignment between the tools and the embryonic fins which begin forming on initial contact between the rolls and the tube. This misalignment results from the fact that the finish discs of large diameter engage the work before the' cutting disc or discs of smaller diameter and from the fact that the discs of different diameter inherently extend to different diameters on the work piece, and hence generate or tend to generate fins of different helix angles. This of course is because at constant pitch the helix angle is a function of diameter.

The troublesome situation in starting the production of extremely fine pitch finned tubing arises in part because of the fact that the fins will be thinner, measured axially of the tubing, the finer the pitch of the tubing. Aside from the necessity to conserve metal, the fins must be made thinner in order that the clearance between adjacent fins will be sutficient to give a geometry that will function properly in service. This aspect is important both to avoid the overlapping of laminar sub-layers when fluids are mov-' ing cross-flow around the fins, and also from the standpoint of condensate retention when the finned tubing is used for condensing vapors. In either case, the heat transfer performance can be seriously affected. For these reasons it is required that as the pitch of a finned tube is made finer, the fins will be made correspondingly thinner.

When the fins are as thin as required for the extremely fine pitch finned tubing contemplated herein, the slight misalignments between the finning tools and the individual discs thereof and the tube, which take place during radial infeed of the rolls as practiced conventionally heretofore, in which the rolls have fixed angles of skew and ttaper, cannot be tolerated. Thin fins are not strong enough in the direction parallel with the axis of the tube to permit the distortion and therefore they strip off, resulting in a garbled contour and the excessive production of spoiled a work or scrap. However, by the method disclosed herein, the misalignment problem in starting is overcome and scrap resulting from spoiled starts may be eliminated even when producing finned tubing of extremely fine pitch.

In accordance with the present invention the finning rolls which are normally driven in rotation are moved radially inwardly of the tubing from a position in clearance with respect to the tubing to a working position at finning depth. During this radial infeed movement of the rolls the skew and taper angles are simultaneously and continuously varied so that the cutter disc or discs of the minimum diameter properly preform the fins which are subsequently developed by the action of the finishing discs of larger diameter and the misalignments which have heretofore led to shearing off of the fine pitch fins is eliminated. It is essential to note that the present invention involves a dynamic or in-process change of the taper and skew angles. It is to be sharply distinguished from the use of heads in which the taper and/or skew angles of the rolls may be adjusted beforehand.

Referring now to the figures, FIGURE 1 shows a fragmentary sectional view through a portion of a finning roll. In this figure it will be observed that there are provided a plurality of discs designated 10, 12, 14, and 16, all of which are of uniform thickness so as to provide a constant pitch. The edges of the discs are shaped so that the discs are of progressively increasing diameter from the initial disc to the final disc 16. In addition, it will be observed that the initial disc 16, which is referred to as a cutter disc, has its peripheral portion shaped to a relatively thin cross-section. The second disc 12, while of slightly greater diameter than the disc 10, also has its peripheral portion shaped so that it too may be referred to as a cutting disc. The discs 14 and 16, which are of increasing diameter, have their peripheral portions shaped more bluntly so as to provide the desired contour to the bottoms of the spaces between the fins on the tube. In this figure a series or stack of four discs is illustrated but it is common to use more than four, and in fact, in FIG- URES 25, the rolls are illustrated as provided with five discs in each stack.

In conventional finning in which the entire length of tube is provided with fins, the end of the tube is advanced into the space between the sets of rolls so that the tube first contacts the cutting disc 10 and thereafter progressively contacts the cutting disc 12 and the finishing discs 14 and 16 and other if provided. However, when it is desired to produce a finned section intermediate a pair of plain or unfinned sections, then the initiation of the finning operation has in the past been accomplished by feeding the rolls radially inwardly with respect to the tube:

Under these circumstances it will be apparent that the first disc of the roll to contact the tube is the disc 16 and that the discs of successively smaller diameter; namely, the discs 14, 12 and 10, will come into operation only as the radial infeed of the tool continues. After the tool has been radially fed to proper finishing depth, then the finning operation continues exactly as in the case where the end of the tube is introduced between the space between the rolls. However, where the operation is carried out to produce fins of the fine pitch disclosed herein, it is found that the transition zone which is formed by the rolls from the interval between initial contact between the rolls and the tube to the arrival of the rolls at finning depth, in many cases results in garbled fins and therefore a scrap work piece.

In the finning operation the plain tube is supported so that it may be rotated about its own axis as a result of the driving engagement of the finning rolls, In addition, the tubeis supported so that it may be advanced axially as a result of the driving engagement of the finning rolls. In order to produce helical fins and also to produce axial advance of the tube, each of the rolls is positioned with its axis oriented with respect to the axis of the tube so as to provide an angle therebetween which is referred to herein as the skew angle. This angle is determined by the pitch of the fin or fins, the number of starts or separate continuous helical fins produced on the work piece, and the diameter of the work piece. It will be observed that the skew angle increases with an increase in fin pitch, decreases with an increase in tube diameter, and is approximately doubled or trebled when changing from a single start fin to a two or three-start fin, respectively.

Referring now to FIGURES 2 and 3 there is illustrated a section of tube T and a roll R, the roll comprising as illustrated a series of five discs 10, 12, 14, 16 and 18, mounted on an arbor 20 which is adapted to be driven in rotation by suitable means (not shown). It will be observed that the successive discs 10, 12, 14, 16 and 18 are of progressively increasing diameter.

As seen in FIGURES 2 and 3, the roll R has been fed radially inwardly with respect to the tube T so that initial contact has been made only between the first or cutting disc 10 of the roll, the remaining discs 12, 14, 16 and 18, as clearly seen in FIGURE 3, remaining in a position of radial clearance with respect to the tube T. It will also be observed that at this time the roll R is so oriented with respect to the tube T that its skew angle, as seen in FIGURE 2 where it is designated as the angle A1, has a positive value but less than the angle required for continuous finning when the roll R has reached full depth as may be seen by comparing FIGURES 2 and 4.

As best seen in FIGURE 3, the orientation of the roll R with respect to the tube T is such as to provide a taper angle B1 measured in a plane at right angles to the plane containing the skew angle A1. The taper angle B1 is such that initial contact between the roll R and the tube T is limited to contact of the initial or cutting disc 10 and the outer surface of the tube T.

Inasmuch as the roll R has a definite skew angle A1 when it initially contacts the tube T, as clearly illustrated in FIGURE 2, it will be apparent that initial engagement between the rolls and tube results in driving the tube in rotation and advancing the tube axially in accordance with the skew angle of the rolls.

In accordance with the present invention the continued radial infeed of the rolls to finning depth is accomplished by a continuous change in both the skew angle and the taper angle.

Reference is now made to FIGURES 4 and 5 which are respectively, plane and elevational views of the roll R and tube T after the roll has arrived at its full finning depth. In this case it will be observed that the skew angle has increased from A1 to A2, this being the angle determined by pitch of the fins, the diameter of the tube, and the number of starts of the helical fin or fins. Furthermore, it will be observed in FIGURE 5 that the change in taper angle is such that when the roll R is at finning depth with respect to the tube T, the taper angle which was initially the angle B1 at contact as shown in FIG- URE 3, has now approached zero although this angle is designated B2 in FIGURE 5. This is for the reason that the taper angle, while it approaches zero, need not actually be a zero angle and instead, may have a small positive value.

After the finning rolls have reached finning depth and the angular orientation illustrated in FIGURES 4 and 5, continued rotation of the rolls results in formation of the fine pitch fins to the full height designed into the rolls and this operation may continue indefinitely dependent on the length of tubing. However, where a short section of finned tubing is to be provided between adjacent plain or unfinned sections, the finning operation is terminated by shifting the rolls radially outwardly into clearance with respect to the tubing and this movement may of course be an abrupt movement.

It is found that the transition Zone at the start of finning may be from one to two inches measured axially of the tube. The transition Zone at the end of the finning operation is of course determined solely by the dimensions of the rolls, measured axially of the tube.

The skew angle A2, which prevails during the finning operation as previously discussed, is an angle whose value may be computed in accordance with fin pitch, tube diameter, and the number of starts of the fin. The initial skew angle A1 must be of a value sulficient to produce appreciable axial advance of the tube and in practice, the angle A1 existing at initial contact between the roll and tube may be approximately one-half A2.

The angle B2 prevailing during the continuous finning operation, as indicated above, may be zero or may be a small positive angle, as for example not ordinarily exceed ing one degree. The initial taper angle B1 at the initiation of contact between the roll R and the tube T will be determined largely by the geometry of the roll and will of course be somewhat greater than the angle between a line tangent to the edge portions or tips of the discs 10, 12, 14, 16 and 18 and the axis of the roll.

In both cases the change in the angles from A1 to A2 and from B1 to B2 is continuous throughout the radial feed of the rolls, and preferably takes place at a uniform rate. This permits the required continuous variation in the skew and taper angles to be properly related to the infeed of the rolls by suitable camming devices.

Referring now to FIGURE 6 there is illustrated the product produced by the practice of the present invention. In this case the tube T is illustrated as provided with plain or unfinned edge portions 22 and 24 between which is the finned portion herein designated 26. In this figure no effort is made to illustrate the transition zone between the unfinned portions 22 and 24 and the intermediate full finned portions of the finned zone or section 26.

The present invention, as illustrated in FIGURE 6, produces fins having a fin height extending radially above the outer surface of the plain end portions 22 and 24. However, it is within the contemplation of the present invention that the finning operation shall be carried out, as is well known in the art, with such reduction of tube diameter as to provide a finned portion in which the crests of the fins are of the same or of a lesser diameter than the diameter of the unfinned tube portions 22 and 24.

By the practice of the present invention it is for the first time possible to produce commercial finned tubing in which the fins have a height from root to crest at least greater than the axial spacing between adjacent convolutions of fins, and in which the frequency of fins is at least 26 fins per inch.

In the actual rolling operation, a mandrel may in some cases be located within the tubes, as is well understood in the art.

The drawings and the foregoing specification constitute a description of the improved fine pitch finned tubing and method of producing the same in such full, clear, concise and exact terms as to enable any person skilled in the art to practice the invention, the scope of which is indicated by the appended claims.

What we claim as our invention is:

1. The method of making fine pitch finned tubing which comprises the steps of providing rotation of a plain tube and a finning roll having a plurality of discs of progressively increasing diameter, providing a relative feed motion between said tube and roll in a direction substantially radial of said tube, and decreasing the taper angle of the roll relative to said tube and increasing the skew angle of said roll relative to said tube during the feed motion.

2. The method as defined in claim 1 which comprises providing a skew angle of the roll relative to the tube upon initial contact between the roll and tube which is less than the appropriate skew angle for continuous fin ning as determined by the pitch, number of fin starts and tube diameter, and increasing the skew angle throughout relative feed motion to provide the aforesaid appropriate skew angle at termination of feed.

3. The method as defined in claim 1 which comprises providing a taper angle at initial contact between the roll and disc such that only the first disc of least diameter contacts the tube initially.

4. The method as defined in claim 3 which comprises decreasing the taper angle throughout relative feed motion to provide a taper angle approaching zero at termination of feed.

5. The method as defined in claim 1 which comprises providing rotation of the roll and tube by rotating one of them directly and the other of them solely through driven engagement with the said one of them.

6. The method as defined in claim 1 which comprises driving the roll directly and driving the tube solely through its engagement with the roll.

7. The method as defined in claim 1 which comprises providing three rolls uniformly spaced around the tube.

8. The method as defined in claim 1 which comprises driving the rolls directly and the tube solely through its engagement with the rolls.

9. The method of forming a section of fine pitch finned tubing intermediate plain section which comprises positioning a plurality of finning rolls with their axes skewed with respect to the axis of the tube and located at equal intervals around a plain tube, the rolls being formed of a series of discs of progressively increasing diameter, driving the rolls in rotation and supporting the tube for rotation and axial advance as a result of its engagement with the rotating finning rolls, feeding the rolls radially inwardly of the tube to proper finning depth, during such feed changing the angular orientation of the rolls to decrease the taper angle thereof from a taper angle at contact with the tube such that only the first disc of least diameter contacts the tube in advance of successive discs, to a taper angle not substantially greater than zero when the roll is at finning depth, continuing rotation of the rolls and tube and axial advance thereof as a result of the skew angle of the rolls, and moving the rolls radially outwardly of the tube to terminate the finned section adjacent a section of plain tube.

10. The method as defined in claim 9 which comprises the step of increasing the skew angle of the rolls continuously during feed motion thereof radially of the tube.

No references cited.

WILLIAM W. DYER, JR., Primary Examiner. G. A. DOST, Assistant Examiner. 

1. THE METHOD OF MAKING FINE PITCH FINNED TUBING WHICH COMPRISES THE STEPS OF PROVIDING ROTATION OF A PLAIN TUBE AND A FINNING ROLL HAVING A PLURALITY OF DISCS OF PROGRESSIVELY INCREASING DIAMETER, PROVIDING A RELATIVE FEED MOTION BETWEEN SAID TUBE AND ROLL IN A DIRECTION SUBSTANTIALLY RADIAL OF SAID TUBE, AND DECREASING THE TAPER ANGLE OF THE ROLL RELATIVE TO SAID TUBE AND INCREASING THE SKEW ANGLE OF SAID ROLL RELATIVE TO SAID TUBE DURING THE FEED MOTION. 